GSH S-benzo(a)pyrene (BP)-4,5-oxide transferase is located in 105,000 g supernatant. The enzyme also makes conjugates with BP-7,8-dihydrodiol-9,10-oxide and BP-9,10-dihydrodiol-7,8-oxide, but not with BP-7,8-oxide, phenols, quinones and dihydrodiols. The enzyme, which produces water-soluble materials by conjugating with GSH, has an implication in the detoxification of chemical carcinogens.